The Formative Stage of the Island
Jeju Island, a volcanic island, was formed from the volcanic activities occurring from the Pliocene Epoch(at the end of the 3rd Cenozoic era) to the Pleistocene Epoch(the 4th Cenozoic era). The volcanic rock of basalt, which constitutes the base of the island, started to pour forth from under the sea. As a result of the continuous volcanic activities during the 4th Cenozoic Era(1.2 million - 2.5 million years ago), Jeju Island is composed of sedimentary layers, particularly in such places as Seongsan, Hwasun, and Shinyang-ri. Jeju Island also consists of several volcanic rocks including basalt, trachytic, andesite and various output scoria from the dormant volcano function. Such volcanic activities on Jeju Island are generally divided into 5 stages, and more than 110 confirmed lava eruptions have occurred.
The First Eruption Stage(1.2 million years ago) – The Formation of the Base
The foundation of Jeju Island is composed of the Seogwipo layer, basal basalt, and indeterminate sedimentary layers. From the Seogwipo formation, which is partly exposed in the southwestern coastal cliff, animal fossils have been found and confirmed as being from the Pliocene Epoch(at the end of the 3rd Cenozoic era, 1.2-2.6 million years ago). Therefore, the basal basalt layer positioned lower can be viewed as being from earlier eruptions. Furthermore, during the drilling for underwater development at various places below the sea surface surrounding Jeju Island, identical basalt layers were confirmed to exist. These layers make up the base of Jeju Island.
The second eruption stage(700,000-1.2 million years ago) – The Formation of Jeju Island
Pyoseonri basalt erupted from the Seogwipo sedimentary stratum and formed the first lava plateau. Thereafter, the eruptions of Seogwipo trachyte and Jungmun trachyte occurred before the sedimentary periods of the Seongsan and Hwasoon stratum. Consequently, Jeju Island was formed centering around the coastal area that links Mt.Sanbang and Seogwipo. As the scoria accumulated, oreums or the parasitic cones took shape including Sunrise Peak(Seongsan Ilchulbong), Mt.Sanbang, Gosan Peak, Byuldo Peak and Dansan.
The Third Eruption Stage(300,000-700,000 years ago) – The Formation of the Coastal Lowlands
At this time, the Hahyori basalt and Beopjeongri trachyte were spewed out with the resumption of volcanic activities after being dormant for about 100,000 years. The lava that poured forth had a low viscosity and as a result, a high liquidity was attained. This allowed the lava to flow widely and form a plateau that covered the edge of most of the coastal areas. For the same reason, even though dozens of eruptions occurred, the height of Mt.Halla around this time is estimated to have been merely about 950 m, well below its present height.
The Fourth Eruption Stage(100,000-300,000 years ago) – The Formation of a Volcano, Mt.Halla
Starting with the eruption of Siheungri basalt, the belching of the trachytic andesite of Mt.Halla occurred during this period. In the early stage, the outflow widely covered the east-west coastal area but its activity gradually became limited to the area around Baeknokdam, the crater on the summit of Mt.Halla. The lava vomited during this period had high viscosity/low mobility; therefore, it could not spread widely and ended up accumulating around the crater. That is how the body of Mt.Halla was built up to its present height.
The Fifth Eruption Stage(100,000-25,000 years ago) – The Formation of the Parasitic Volcanoes
During this time, parasitic volcanoes called "oreums" were formed by simultaneous eruptions around the foot of Mt.Halla. First, at the center of Mt.Halla, Baengnokdam basalt erupted, breaking up the trachytic top of the northeastern side of Mt.Halla so that a basaltic hole was made. With this event, Mt.Halla took on the shape we see today. At the same time, the vigorous volcanic activities around Mt.Halla led to the current configuration and the unique geographical features of Jeju Island.